Casting readily oxidizable alloys



2,770,860 CASTING READILY OXIDIZABLE ALLOYS Fred J. Webbere, Royal Oak, Mich, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application July 23, 1952,

- Serial No. 300,512

7 Claims. (Cl. 22-214) This invention relates to metal casting operations and particularly to a process and atmosphere for use in eliminating certain difficulties encountered in casting alloys which contain readily oxidizable components. When cast in air, certain alloys exhibit a tendency to form oxide and/or nitride films which fold into the surface of the metal, resulting, in most instances, in visible blemishes and planes of vweakness in the final casting. For'example, the presence of aluminum or titanium in many alloys introduces a serious problem in casting such alloys because these elements readily form refractory oxides. In pouring such a molten alloy through air, a

thin. tenacious oxide filmforms on the exposed surface of the metal stream. Heretofore many attempts to cast this ,type of alloy were unsuccessful due to the folding-in or entrapment of thisoxidefilm in the cast metal. The occurrence of a sizable fold results in a plane of weakness since there is little cohesive strength across such adefect. Even very small inclusions of this type are considered to be stress-concentrators and should. be avoided in any parts which are subjected to fatigue.

Such internal folds are not readily detectable byX-ray methods and, inasmuch as the actual size of a fold cannot be accurately predicted by the amount exposed at the cast surface, it is necessary-to employ a casting procedure which eliminates or at least minimizes the occurrence of oxide folds in castings which are to be used under high stress. In accordance with my invention, therefore, a special casting technique is employed which eliminates the above-described difficulties and provides sound castings which are especially adapted for use under high stressconditionasuch as for turbine buckets.

It will be understood, of course, that the results obtained are dependent upon many interrelated variables, such as melting temperature, pouring rate, method of gating, size of melt, etc. It is therefore difficult to completely eliminate minor folds from every casting.- However,-the procedure described herein has permitted the production of sound castings of alloys which, when cast by procedures heretofore used, yielded only defective castings.

The quantity of the oxide films on the surface of the melt often can be reduced by melting under a suitable neutral atmosphere, such as helium or argon. Air can be excluded during the pouring operation by clamping the mold directly to the furnace and maintaining a slight positive pressure ofthe inert gas over the melt during the pour. In many instances, however, this precaution may not be. effective due'to the occlusion of gases near the mold surfaces. Moreover, the use of argon or helium alone is unsatisfactory because these gases are not sufliciently heavy to ,settl'ein the furnace during the interval that the mold is being transferred to the furnace and hence will allow contamination of the furnace atmosphere.

Acct; rd in gly, a principal object of the presentinvention is to provide an atmosphere in which metals containing readily oxidizable constituents may be successfully cast.

2,770,860 Patented Nov. 20,1956

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2 A further object of this invention is to provide a process for vapor fluxing with such an atmosphere so as to form castings in which the aforementioned oxide films are eliminated or greatly reduced.

These and other objects are attained in accordance with my invention by the use of a particular type of nonoxidizing gas atmosphere during the casting operation. I have found that vapor fluxing during the pouring operation with gases or vapors containing fluorine is a generally satisfactory method of eliminating the above-de-- and poured in air. Of the above-mentioned fluorocarbons, dichlorodifiuoromethanehas provided the most outstanding results. I

In practicing the present process in a preferred manner the active fiuxing gas or vapor is introduced along with a dry inert gas, such as helium or argon, above the surface of the melt for a short period of time before clamping the mold to the furnace. After' the mold is clamped in position, a short purge with the active gas follows.

As an illustration of a specific application of my invention, master heats of the alloy to be cast may be first compounded and solidified in a form suitable for making up small charges for remelting. Bar stock, small ingots or shot can be ,used, and both high frequency induction furnaces and indirect arc furnace have been successfully employed. In each instance" the furnace should. be equipped with an appropriate tube'for introducing the gas above the surface of the melt, and the melting furnace should be lined with magnesia orfa suitable refractory material. The lining in an induction furnace is usually a preformed crucible. v a

It is preferable to have thefurnace constructed so that the pouring basin of the mold canbe' clamped directly over the pouring lip of the furnace. Likewise, it'is desirable to mount the furnace on trunnions so thatith e mold and the furnace can be inverted as a unit; During the actual pouring step, the flow of metal is as smooth as possible, with a minimum of turbulence and the avoidance of any substantial drop as the metal flows into the mold.

To more effectively prevent-the formationand occlusion of metal oxide and nitride films, the melting time should be as short as possible, and closer temperature 'control-is possible if the surface of the melt is free of oxides. In most instances the film on the melt will break as the temperature is increased, but in the most stubbornca'ses the film can be broken by the addition of a-small amount of a strong reducing agent such as calcium-silicon, a. quantity in the order of 0.05% by weight being sufiicient.

For best results, an atmosphere of the inert gas should be maintained in the furnace during the melting operation, argon being preferable to helium in most instances. As an example of the flow rate which should be used, an argon flow rate of 5 to 2 0 cubic feet per hour issuitable for furnaces having capacities in the rangebetween ito As hereinbefore indicated, the fluxing gas, such as dichlorodifluoromethane, is introduced into'the argon or helium stream immediately before positioning the mold on the furnace. An initial flow rate of about 5 cubic feet per hour may also be used for the dichlorodifluoromethane for a purge of approximately 5 to seconds. After this brief purge with the mixture of the inert and active gases, the mold is placed in position and the purging continued for a short length of time, usually in the order of 5 to 30 seconds, to allow the air in the mold cavity to become completely displaced. During this latter period, the flow rate of the dichlorodifiuoromethane is preferably increased to about 10 cubic feet per hour for about 10 to seconds. Hence, during the purge, the flow rate of the active gas preferably should be maintained between approximately and 70% of the combined flow rate of the two gases.

The assembly is then tipped at a steady rate, transferring the molten metal from the furnace to the mold cavity. It is usually advisable to stop the flow of the argon and dichlorodifluoromethane as the tipping begins.

The use of a fluorinated hydrocarbon gas, such as dichlorodifluoromethane, provides a heavy atmosphere in the furnace which is not readily displaced by air. Dichlorodifluoromethane is, in addition, a reactive gas at the temperature of the melt, and the chlorine and the fluorine form a flux which is believed to render any film on the surface of the melt less subject to rupture and fragmentation during pouring. Liquids also can readily be used as the fluxing medium in this process by saturating the neutral carrier gas with the active vapor, an ordinary wash bottle being convenient for this purpose.

The gaseous fluxes heretofore employed for such melting and casting operations contain many disadvantages. Not only are unsatisfactory results obtained due to the formation of oxide or nitride films, but the use of many of these fluxes is also objectionable from the standpoint of cost, availability, presence of toxicity at room temperature, and commercial purity. 0n the other hand, dichlorodifluoromethane is an inexpensive gas which is not toxic at room temperature and'which is readily commercially available in highly pure form. Moreover, the use of many of the chlorinated hydrocarbons heretofore used, such as methyl chloride, introduces additional problems because of hydrogen absorption by the casting metal, and the amounts of these chlorinated hydrocarbons must be regulated so carefully as to make their use impracticable.

As hereinbefore indicated, I have found that the abovedescribed process is especially useful in casting certain readily oxidizable alloys which recently have been successfully used in forming parts such as turbine buckets and nozzle diaphragm vanes for gas turbines. An example of this alloy is a nickel-base alloy containing approximately 1.5% to 3% titanium and 2% to 4% aluminum.

While my invention has been described by means of certain specific examples, it will be understood that the scope of the invention is not to be limited thereby except as defined in the appended claims.

I claim:

l. A process for melting and casting an alloy containing readily oxidizable constituents which comprises melting the alloy under an inert atmosphere and casting said alloy under an atmosphere consisting essentially of an inert gas selected from the group consisting of argon and helium and a halogenated hydrocarbon containing at least one fluorine atom and one chlorine atom.

2. A process for casting a readily oxidizable molten metal which comprises casting said metal in an atmosphere consisting essentially of at least one dry inert gas selected from the class consisting of argon and helium and at least one member selected from a class consisting of dichlorodifluoromethane, carbon tetrafluoride, trifluoro- 4 bromomethane, trichlorotluoromethane, trifluorochloromethane and trichlorotetrafluoroethane.

3. A process for melting and casting a molten metal containing readily oxidizable constituents, said process comprising melting the metal in a furnace under an atmosphere of an inert gas selected from the group consisting of argon and helium, purging the atmosphere surrounding said metal with a fluorinated hydrocarbon, thereafter immediately positioning a mold on the furnace, continuing said purge for a short period of time to displace the air in the mold cavity, and thereafter pouring the molten metal into the mold.

4. A process for casting a molten metal containing readily oxidizable constituents, said process comprising melting the metal in a furnace while under an atmosphere of an inert gas selected from the group consisting of argon and helium, purging the atmosphere surrounding said metal by introducing into the inert gas atmosphere a gaseous halogenated hydrocarbon containing not more than two carbon atoms and at least one fluorine atom and one chlorine atom, thereafter immediately positioning a mold on the furnace, continuing the purge with the inert gas and halogenated hydrocarbon for a short period of time to displace the air in the mold cavity, and thereafter pouring the molten metal into the mold.

5. The process of casting an alloy containing readily oxidizable constituents, said process comprising melting the alloy in a furnace under an atmosphere of a dry inert gas selected from a class consisting of argon and helium, introducing dichlorodifluoromethane into the inert gas stream to purge the atmosphere surrounding the molten metal, the flow rate of said dichlorodifluoromethane being maintained between approximately 20% and 70% of the combined flow rate of the inert gas and the dichlorodifluoromethane, subsequently immediately positioning a mold on the furnace, thereafter continuing the purge for five seconds to thirty seconds to permit the air in the mold cavity to be completely displaced, and finally pouring the molten metal at a steady rate into the mold cavity.

6. A method of treating a nickel base alloy containing titanium and aluminum during pouring operations so as to prevent the formation and occlusion of titanium and aluminum nitride and oxide films, said method comprising melting said nickel base alloy and pouring it in an atmosphere consisting essentially of a mixture of an inert gas and dichlorodifluoromethane.

7. A method of treating a nickel base alloy containing approximately 1.5 to 3% titanium and 2% to 4% aluminum during pouring operations to prevent the formation and occlusion of titanium and aluminum nitride and oxide films, said method comprising melting said nickel base alloy and pouring it in an atmosphere consisting essentially of 20% to 70% by volume of a halogenated hydrocarbon containing not more than two carbon atoms and at least one fluorine atom and one chlorine atom and the balance substantially all at least one dry inert gas selected from the class consisting of argon and helium.

References Cited in the file of this patent UNITED STATES PATENTS 

6. A METHOD OF TREATING A NICKEL BASE ALLOY CONTAINING TITANIUM AND ALUMINUM DURING POURING OPERATIONS SO AS TO PREVENT THE FORMATION AND OCCLUSION OF TITANIUM AND ALUMINUM NITRIDE AND OXIDE FILMS, SAID METHOD COMPRISING MELTING SAID NICKEL BASE ALLOY AND POURING IT IN AN ATMOSPHERE CONSISTING ESSENTIALLY OF A MIXTURE OF AN INERT GAS AND DICHLORODIFLUOROMETHANE. 